Do raisins need a Hechsher?
Raisins are often treated after production with oil, and/or glycerin which can come from non-Kosher sources.
Also, in the process of drying, often fatty esters are used, creating a Kashrus concern, as to the source of the esters. Some say that non-golden unflavored raisins produced in the US are free of these concerns.
Another serious issue is the infestation all too common in raisins. Some of the infestation can occur when drying in open fields with insects being attracted to the raisins. During prolonged storage too, there can be infestation. Even if oven dried, there is often infestation. Of course, some infestation could happen while the grape is still attached to the ground. All this has bearing of the actual Halacha. One would also need to determine how often this would happen and whether this exceeds the threshold of Miut Hamatzu, “a minority which is common”, which requires checking according to Halacha. There are still other aspects of leniency, based on various doubts, see Taz YD 84:12.
Based on all the above, one should only purchase Kosher L’mehadrin raisins, which are made from currants and worm free, or sample checked for infestation by a Mehadrin Kashrus agency and recheck before usage.